Reduction of surface recombination current in GaAs devices

ABSTRACT

Surface recombination current in GaAs devices is reduced by means of a semi-insulating, oxygen, iron or chromium doped monocrystalline layer of AlGaAs grown by MBE. The AlGaAs layer is grown on a GaAs body and is then masked. Diffusion of suitable impurities through a window in the mask converts the exposed portions of the AlGaAs layer to low resistivity and modifies the conductivity of the underlying zone of the GaAs body. The peripheral portions of the AlGaAs layer, however, remain semi-insulating and are effective to reduce the surface recombination velocity - diffusion length product by more than an order of magnitude.

BACKGROUND OF THE INVENTION

This invention relates to GaAs devices and, more particularly, to thereduction of surface recombination current in such devices.

In the area of Group III-V compound semiconductors recent investigationof the current-voltage behavior of Al_(x) Ga_(1-x) As--Al_(y) Ga_(1-y)As--Al_(x) Ga_(1-x) As (0≦y<x), double heterostructure, p-n junctiondevices has shown that surface recombination is responsible for theobserved 2kT current. This surface recombination current is due tononradiative electron-hole recombination in the surface depletion regionat the junction perimeter. Earlier studies of surface current had beenconcerned with GaP₀.4 As₀.6 and GaP light-emitting diodes where thesurface current was also found to be the dominant 2kT current. Etchingthe GaP surface in a CF₃ plasma did reduce the surface current. However,desorption of F resulted in reversion to the etched surface values. Insilicon technology, on the other hand, growth of SiO₂ on Si can greatlyreduce the surface recombination current. However, surface chemicaltreatment or the growth of native oxides, SiO₂ or silicon nitride onGaAs has not been helpful, and the reduction of the surfacerecombination current on GaAs has eluded solution.

SUMMARY OF THE INVENTION

In accordance with an illustrative embodiment of our invention, thesurface recombination current in a GaAs device is reduced by a processwhich includes epitaxially growing by molecular beam epitaxy (MBE)semi-insulating Al_(x) Ga_(1-x) As monocrystalline layer (x≳0.4preferably) on a major surface of a GaAs monocrystalline body,depositing a suitable masking layer, such as silicon nitride, on theAlGaAs layer, opening a window in the masking layer to expose a portionof the underlying AlGaAs layer but leaving adjacent portions of theAlGaAs covered by the masking layer, diffusing impurities through thewindow, through the AlGaAs layer and into the underlying zone of theGaAs body, thereby substantially decreasing the resistivity of theexposed portion of the AlGaAs layer and modifying the conductivity ofunderlying zone of the GaAs body, and forming electrical contacts to thenow low resistivity portion of the AlGaAs layer and to the GaAs body.

The diffused impurities may modify the conductivity of the underlyingzone of the GaAs body in several ways well known in the art. Acceptorsdiffused into an n-type GaAs body may result in a p-type or compensatedzone under the window and hence a p-n junction at the diffusionboundary. In a similar fashion, donors may be diffused into a p-typeGaAs body to form a p-n junction. Devices incorporating such junctionsmay function in various ways, for example, as light-emitting diodes,photodiodes, or field effect transistors. Alternatively, donors oracceptors may be diffused into a GaAs body of the same conductivity typeto increase the cnductivity of the zone of the GaAs body under thewindow. This procedure might be useful in facilitating the formation ofohmic contacts, for example. In each case above, however, the diffusionof impurities converts the portion of the semi-insulating AlGaAs layerin the window to a low resistivity layer, thus permitting the formationof low resistance electrical contacts. However, the adjacent peripheralportions of the AlGaAs layer under the mask remain highly resistive and,in accordance with one feature of our invention, serve to significantlyreduce surface recombination current. In one example of the aboveprocess an oxygen-doped AlGaAs semi-insulating layer was used to reducethe intrinsic surface recombination velocity--surface diffusion lengthproduct of a Zn-diffused p-n junction GaAs device by more than an orderof magnitude from 3.7 cm² /sec to 0.3 cm² /sec. Other dopants such asiron and chromium, also render AlGaAs semi-insulating and, we believe,would also be effective in reducing surface recombination current.

In addition, the masking layer, illustratively silicon nitride, remainson the end product and provides protection from the environment so thatthe combination of nitride-AlGaAs meets the requirements of GaAs surfacepassivation.

BRIEF DESCRIPTION OF THE DRAWING

Our invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing in which the sole FIGURE is across-sectional, isometric view of a GaAs semiconductor devicefabricated in accordance with an illustrative embodiment of ourinvention. For the sake of clarity, the device is not drawn to scale.

DETAILED DESCRIPTION

With reference now to the drawing, a GaAs semiconductor device is showncomprising a GaAs monocrystalline body 10 which may include, forexample, a substrate 12 and an epitaxial layer 14. A monocrystalline,semi-insulating Al_(x) Ga_(1-x) As layer 16 (x≳0.4 preferably) is grownon one major surface of body 10 (or epitaxial layer 14) and a dielectricmasking layer 18 is deposited on layer 16. Layer 16 is typically dopedwith oxygen but iron and chromium may also be suitable, Preferably,layer 16 is thick enough (e.g., >1000 Angstroms) to prevent anyappreciable tunneling therethrough. Layer 18 has a generally circularwindow therein, and an annular metal electrode 20 is formed in thewindow in contact with an annular portion of layer 16 exposed by thewindow. Electrode 20 includes an appendage 22 to which an electricallead or conductor (not shown) may be connected. Appendage 22 does not,however, contact AlGaAs layer 16. A metal layer on the opposite majorsurface of body 10 forms a counter electrode 24.

Body 10 includes a diffused zone 26 in substantial registration with thewindow. Localized zone 26 has its conductivity modified with respect tothe remaining portions of the body. A feature of the device is that theperipheral portions 16.1 of the AlGaAs layer 16 (i.e., those undermasking layer 18) have a relatively high resistivity and are effectiveto significantly reduce surface recombination current, but the centralportions 16.2 in the window have much lower resistivity so as tofacilitate making good ohmic contact to the upper major surface of body10.

As discussed previously, zone 26 may have either n-type, p-type, orcompensated conductivity so that boundary 28 between zone 26 and theremainder of body 10 may be either a p-n junction, an n-n⁺ interface ora p-p⁺ interface. When boundary 28 is a p-n junction, the device canfunction as a light-emitting diode under forward bias or as a photodiodeunder reverse bias with light emitted or detected propagating throughthe window in layer 18. On the other hand, when boundary 28 is n-n⁺ orp-p⁺, then zone 26 would typically have higher conductivity than theadjacent portions of body 10 so as to facilitate making good ohmiccontact to body 10 as mentioned above. A pair of spaced zones 26 and awell-known gate electrode configuration (not shown) may be used as afield effect transistor.

The fabrication of the GaAs device starts with obtainin a GaAs substrate12 from commercial sources. If desired, an epitaxial layer of GaAs 14,of the same conductivity type as substate 12, may be grown thereon byany of several well-known growth techniques including LPE, VPE or MBE.Next, MBE is used to grow a semi-insulating, monocrystalline, Al_(x)Ga_(1-x) As layer 16 on the upper major surface of body 10 as describedby H. C. Casey, Jr., A. Y. Cho, and E. H. Nicollian in copendingapplication, Ser. No. 869,369, filed on Jan. 13, 1978, and assigned tothe assignee hereof. Layer 16 is typically doped with oxygen during MBEgrowth but iron and chromium may also be suitable in achieving itssemi-insulating character. Preferably, x≳0.4 so that layer 16 is anindirect gap, and hence low mobility, material. By semi-insulating wemean a resistivity preferably in the range of about 10⁸ to 10¹² Ω-cm.Layer 16 also thick enough to prevent any appreciable tunneling and isillustratively several thousand Angstroms thick.

A dielectric masking layer 18, such as silicon nitride, is thendeposited on layer 16 and, using standard plasma etching orphotolithographic techniques, a window is opened therein to expose theunderlying portion 16.2 of layer 16. A dopant (e.g., Zn) is thendiffused through the window to modify the conductivity of body 10 inzone 26 (e.g., to form a p-n junction 28) and, at the same time, toconvert the portion 16.2 of AlGaAs layer 16 from high resistivity to lowresistivity (e.g., about 10⁻¹ to 10⁻³ Ω-cm). While the dopant atoms dodiffuse laterally to some small extent, by and large the peripheralportions 16.1 of layer 16 remain highly resistive and thus reducesurface recombination current.

Electrodes 20 and 24 are formed by standard masking and depositionmethods (e.g., vacuum evaporation or electroplating).

EXAMPLE

The following example is given by way of illustration only. Unlessotherwise stated, therefore, device parameters, growth conditions andthe like should not be construed as limitations on the scope of theinvention.

An n-type GaAs (100)-oriented substrate 12 doped with Si to about 10¹⁸cm⁻³ was obtained from commercial sources. In a standard MBE ultra-highvacuum system the following layers were epitaxially grown on thesubstrate: 1 μm thick buffer layer (now shown) of GaAs doped n-type withSn to about 2×10¹⁸ cm⁻³ ; an n-type GaAs layer 14 about 3 μm thick andalso doped with Sn to about 5.4×10¹⁶ cm⁻³ ; and an oxygen-doped Al₀.5Ga₀.5 As layer 16 about 2000 Angstroms thick.

Both to mask the diffusion and to provide protection from theenvironment, a 1000-1500 Angstrom thick layer 18 of pyrolytic siliconnitride was deposited at 740 degrees C. Circular windows were opened byplasma etching, and a p-n junction 28 at a maximum depth of 2.5 μm wasformed by Zn diffusion at 650 degrees C. for 4 h with a Zn/Ga/As source.The exposed portions 16.2 of the high resistivity Al₀.5 Ga₀.5 As layerwere converted to low resistivity by the incorporation of a highacceptor (Zn) concentration. Ohmic contact 20 to the portion 16.2 (andto zone 26) was obtained by the evaporation of a thin layer of Cr andthen Au over the window--for simplicity an annular contact as shown inthe FIGURE was not fabricated. Contact 24 to the n-type substrate 12 wasmade by evaporated and alloyed Sn-Pt-Sn.

Current-voltage (I-V) measurements were made on the resulting p-njunction device which had a junction area of about 2.8×10⁻⁴ cm². Thecurrent I varied as I=I_(o) exp (qV/nkT) with n=2.0 for V<0.9 V and withn=1.16 at a higher bias voltage. There was additional leakage below10⁻¹⁰ A, although the current was only 10⁻¹² A at 0.2 V. The reverse I-Vcharacteristic was also measured. Near -2 V the current density was only˜1×10⁻⁹ A/cm² and at -8 V the current density was ˜1×10⁻⁸ A/cm². Both ofthese values are very low current densities. Above -8 V the current roserapidly due to microplasmas. In the low current region below -8 V, thecurrent was found to be sensitive to the pressure of the mechanicalprobe. To reduce such effects an electrode configuration with a contactpad (e.g., the appendage 22) separate from the junction region may beused.

The forward bias I-V characteristics were measured for four differentjunction areas (a) between a=8.1×10⁻⁵ and 2.6×10⁻³ cm². To permitcomparison with similar GaAs p-n junctions without the oxygen-dopedAl₀.5 Ga₀.5 As layer, the Al₀.5 Ga₀.5 As layer was removed by etching inwarm HCl before depositing the silicon nitride. A significant differencein the I-V characteristics with and without the oxygen-doped Al₀.5 Ga₀.5As layer were illustrated by plotting the current at a fixed voltage asa function of the perimeter diameter (d) at the surface. For 1.0 V, thecurrent for the case with the oxygen-doped Al₀.5 Ga₀.5 As layer variedas d², while the current for the case without the oxygen-doped Al₀.5Ga₀.5 As layer varied linearily with d. The d² dependence indicates thedominance of bulk current because in this case the current isproportional to junction area. The linear d dependence indicates adependence on perimeter and thus the dominance of surface recombinationcurrent. For 0.8 V, the current for the case with the oxygen-doped Al₀.5Ga₀.5 As layer varied more rapidly than linearily, but less than d²,while current for the case without the oxygen-doped Al₀.5 Ga₀.5 As layerstill varied linearily with d. These p-n junction devices were bright inelectroluminescence at low current (˜1 mA) because the minority carrierlifetime τ was near the limit given by radiative recombination, and thesurface recombination current was negligible.

At V<0.9 V, the 2kT current dominated. The variation of the measuredcurrent with d was measured and used to determine s_(o) L_(s), theintrinsic surface recombination velocity-diffusion length product.Calculations demonstrated that the surface recombination current was adecreasing part of the total current as the area increased and thats_(o) L_(s) =0.3 cm² /sec for a device with an oxygen-doped As₀.5 Ga₀.5As layer as compared to a value of 3.7 cm² /sec for a similar devicewithout the Al₀.5 Ga₀.5 As layer where I varies linearly with d. Theoxygen-doped Al₀.5 Ga₀.5 As layer reduced s_(o) L_(s) by a factor of 12and permitted te n=1 bulk diffusion current to dominate for currentdensities in excess of 5×10⁻³ A/cm².

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

What we claim is:
 1. A GaAs device with reduced surface recombinationcurrent comprisinga monocrystalline GaAs body (10) having a localizedzone (26) of conductivity different from adjacent portions of said body,a monocrystalline layer (16) of oxygen-doped AlGaAs on a major surfaceof said body, said AlGaAs layer having a low resistivity central portion(16.2) in substantial registration with said zone and a semi-insulatingperipheral portion (16.1) surrounding said central portion, a dielectricpassivation layer (18) covering said peripheral portion of said AlGaAslayer, a first electrical contact (20) to said central portion of saidAlGaAs layer, and a second electrical contact (24) to said body.
 2. Thedevice of claim 1 wherein said localized zone is of one conductivitytype and said adjacent portions of said body are of the oppositeconductivity type.
 3. The device of claim 2 wherein said zone containsZn impurities to render it p-type.
 4. The device of claim 1 wherein saidlocalized zone has a higher conductivity than said adjacent portions butis of the same conductivity type as said adjacent portions.
 5. Thedevice of claim 1 wherein said peripheral portion of said AlGaAs layerhas a resistivity in the range of about 5×10⁸ to 5×10¹² Ω-cm and saidcentral portion has a resistivity of the order of 10⁻¹ to 10⁻³ Ω-cm. 6.The device of claim 5 wherein said AlGaAs layer is a direct gap materialand has a thickness sufficient to prevent any substantial tunnelingtherethrough.
 7. The device of claim 4 wherein said dielectric layercomprises silicon nitride.
 8. The device of claim 1 wherein said bodycomprises a GaAs substrate and a GaAs epitaxial layer grown on a majorsurface of said substrate, and wherein said zone is located in saidepitaxial layer.